This invention is related to improving the control of a refrigerator that sequentially operates with two different evaporator temperatures.
Existing refrigerators that are now in production almost exclusively use a single evaporator to cool both the fresh-food and freezer compartments. These systems effectively use the same evaporator temperature to cool both compartments.
This is a simple approach but it results in an inherent efficiency loss. The problem is that the evaporating temperature must be low enough to cool the freezer while a much higher evaporating temperature could be possible for the fresh-food compartment. Theoretically a reduction of 20% or more in energy use is possible for the refrigerator if a higher temperature refrigerant could cool the fresh-food compartment.
There are several possible solutions that can create a second evaporating temperature. One way is to simply have a second circuit. The problem with this approach is that using two small compressors in place of a single large compressor adds a large cost and efficiency penalty.
A better approach is described in U.S. Pat. No. 5,406,805, "Tandem Refrigeration System." This system uses two forced-convection evaporators in series. This system runs only one evaporator fan at a time as a way of creating two evaporating temperatures. The sequence of operation is for the fresh-food evaporator to run first, then the freezer evaporator. Measured energy savings of 10 to 20% have been achieved.
The tandem system relies on the store of liquid refrigerant in the evaporators at start up to provide cooling to the fresh-food compartment. The problem is that a conventional capillary tube has essentially "choked" flow, which means that the mass flow rate is effectively fixed for a given condenser condition. Unfortunately the compressor mass flow increases rapidly with higher evaporating temperatures because of the higher suction gas density. Since the capillary tube has to be sized to handle the flow to the freezer, it is much too restrictive for steady-state operation at a higher evaporator temperature. This limitation means that the tandem system cannot run for more than roughly one or two minutes in cooling the fresh-food compartment without the evaporating temperature approaching that found in the freezer. This limitation also means that the fresh-food evaporator and fan need a large capacity to do all the required cooling in a very short time period.
A similar approach is described in U.S. Pat. No. 5,732,561. This patent uses a damper arrangement to direct air flow to each compartment from a single evaporator. Like the tandem system, it relies on a transient effect to cool the fresh-food compartment. This feature means that the system must first cool the fresh-food compartment, then immediately cool the freezer in order to achieve any energy savings.
An alternate evaporator configuration is described in U.S. Pat. No. 5,867,994 entitled "Dual-Service Evaporator for Refrigerators." This invention uses a reversing fan and flaps that act as check valves to control air flow to each compartment. While this approach simplifies the evaporator design, it does not address the fundamental limitations associated with the refrigeration circuit.
One related problem with these approaches is the compressor sizing. The compressor should be smaller than that for a conventional system to achieve optimum efficiency in these systems, but that limits the pulldown capability. This feature creates the need to trade off efficiency and performance.
Recent developments in compressor technology have created high-efficiency variable capacity compressors suitable for use in refrigerators. The Sunpower linear compressor is a prime example. It has a higher full-load efficiency and can modulate capacity by simply varying the input voltage to the linear motor. The projected production costs for these new compressors are comparable to that of conventional fixed-capacity compressors. Other variable-capacity compressors include those with Electronically Commutated Motors (ECM). Two-speed compressors and compressors with various unloading mechanisms are also possible and found in the prior art.
Results of tests with variable-capacity compressors have been disappointing; there is little or no efficiency improvement at lower flow. A problem with applying the new variable-capacity compressors is that existing systems are not adapted to varying refrigerant flows. The capillary tube and fans have fixed capacity. Changing to variable-speed fans and expansion valves would add a significant cost penalty to the system.